Time-of-flight mass spectrometer



Aug. 7, 1956 w. E. GLENN, .JR 758,214

TIME-oF-FLIGHT MAss SPECTROMETER Filed Dec. 16, 1952 3 Sheets-Sheet 1 TO VACUUM SYSTEM TTORNE Y.

Aug. 7, 1956 w. E. GLENN, JR

TIME-OFF`LIGHT MASS SPECTROMETER 3 Sheets-Sheet 2 Filed Dec. 16, 1952 .n MJ, um AE EL VG me.

M m ...L u w ATTORNEY Aug. 7, 1956 Filed Dec. 16. 1952 W. E. GLENN, JR

3 Sheets-Sheet 5 i'IIIIIL 26 /3 /6 d3.) 27 /9 I f 34 //7 ,26 32 4 NIJ-Z 7/ 55 2z I 5/ a@ I P/ 2P? j @I 5@ N74 @S0 76 E 96 gv.; lX SAwTooTH 73 I GENERATOR PULSE OSCILLATOR FORMING CIRCUIT ACCELERATING VOLTAGE OUTPUT REPELLER GRID VOLTAGE IIIIIIIPLIIJ INVENTOR. WILL/AM E. GLENN, Jr.

TTORNEI.

TllViE-OF-FLIGHT MASS SPECTROMETER William E. Glenn, Jr., Schenectady, N. Y., assigner to the United States of America as represented by the United States Atomic Energy Commission Application December 16, 1952, Serial No. 326,288

9 Claims. (Cl. Z50-41.9)

The present invention relates to an improved mass analyzer and in particular to an electrostatic time-of-ight mass spectrometer having improved resolution and transmission.

There have been developed and employed both electromagnetic and electrostatic mass analyzers; however, to

date the former has found much wider use despite numen' ous advantages of the latter. Thus, entirely electrical mass separators have the advantage of lighter weight, less cost and greater compactness, but in the past has been limited by low transmission, i. e., small percentage of projected ions received or collected, and the difficulty in providing high resolution. The present invention overcomes these diiculties to provide an improved mass spectrometer operating wholly electrically and including a novel and advantageous ion receiver arrangement wherein ions of a single mass are collected to the exclusion of other ions traveling therewith.

It is thus an object of the present invention to provide an improved mass spectrometer accelerating groups of ions of various masses and including means for collecting ions of a single mass therefrom.

It is another `object of the present invention to provide an improved mass spectrometer transmitting an ion beam comprising groups of ions which separate according to their ionic masses and accelerating selected portions of the ion beam corresponding to ions of a single mass for collecting ions so accelerated.

It is still another object of the present invention to provide an improved time-of-ight mass spectrometer having means repelling ions therein from an ion collector and means for selectively accelerating predetermined portions of a bunched ion beam to propel ions of a single mass onto the ion collector.

It is a further object of the present invention to provide an improved mass spectrometer having an ion collector protected from bombardment by any particles except ions of a predetermined mass.

Various other objects and advantages of the invention will become apparent to those skilled in the art from the following description of the invention taken together with the enclosed drawing illustrating a preferred embodiment of the invention.

In the drawing:

Figure l is a longitudinal cross-section of a time-offlight mass spectrometer constructed in accordance with the principles of the present invention;

Figure 2 is a transverse cross-sectional view taken at 2 2 of Figure l;

Figure 3 is a transverse cross-sectional view taken at 3-3 of Figure l;

Figures 4 and 5 `are expanded views of the grid structures at 4 and 5, respectively, of Figure l;

Figure 6 is a schematic representation of the device of Figure l including electrical circuits therefor; and

Figure 7 is a schematic representation and wiring diagram of an alternative collector arrangement.

Considering now the invention, it is noted that time-offlight mass spectrometers commonly operate to ionize a material to be analyzed or separated and to produce an ion beam thereof. The ion beam may be hunched by suit able means into a series of ion bunches, and during travel of this beam the ions in each bunch separate according to their ionic mass as a result of their having a velocity dependent upon such mass. The present invention oper ates in -a similar manner to produce such an ion beam and, in addition, operates upon the beam to collect therefrom only ions of a predetermined mass.

With regard to the elements and structure of the preferred embodiment of the invention illustrated, there is provided an envelope 11 which is adapted to contain an ion beam passing from an ion transmitter 13 at one end of the envelope to an ion receiver 14 disposed at the opposite end thereof. Envelope 11 is open-ended and there is disposed in closing relation to the ends thereof a pair of boxes 16 and 17 which are secured to flanges about the ends of envelope 11 by bolts and nuts, and the assembly is maintained vacuum-tight by suitable gaskets inserted in these connections. Envelope 11 and attached communicating boxes 16 and 17 are evacuated through a vacuum connection 1S extending through a wall of envelope 11 and adapted for attachment to a vacuum pumping system, not shown. Ion transmitter 13 and receiver 14 are disposed within boxes 16 and 17, respectively, and are thus located in a vacuum.

The ion transmitter comprises an ion source 19 and ion accelerating means 21 which are combined in a single unit, as shown. The ion source 19 may consist of a filament wire 22 adapted to be coated with a material to be ionized and connected by leads 23 to rigid conductors 24 extending in insulated relation through the ange portion of envelope 11 to the exterior thereof and terminating at terminals 26. Filament wire 22 is slidably `mounted in an insulating plate 27 which, in turn, is mounted upon a conducting plate 28 having a central aperture therein. Filament wire 22 is deformed into a somewhat lJ-shape and the straight emissive surface thereof is disposed within the aperture in plate 2S in alignment with the front face of plate 28. Plate 28 is mounted upon a plurality of standards 29 which extend axially of envelope 11 into box 16 and are rigidly connected to a face plate 31 connected across the end of envelope 11. Ion source 19 is thus disposed upon an extension `of the axis of envelope 11 with iilament wire 22 being aligned with said axis.

Ion accelerating means 21 includes an accelerating electrode 32 disposed across the end of envelope 11 exterior to face plate 31 and mounted upon standards 29 perpendicular to the axis of envelope 11. Accelerating electrode 32 has a central aperture therethrough about an extension of the axis of envelope 11 and is connected by suitable leads to a lead-through insulator extending through the flange about envelope 11 to comprise an external electrical terminal 33. It is particularly important 'in time-of-ight mass spectrometry to accelerate the ions linearly in a predetermined direction and thus fringing fields and radial iield components must be eliminated. To this end there is provided a plurality of teld rings 34 taking the form of centrally apertured electrodes, as shown in Fig. l, and disposed parallel to accelerating electrode 32 intermediate same and filament wire 22. Field rings 34 are mounted upon standards 29 in insulated relation thereto, and cylindrical insulating spacers are disposed between all members mounted upon standards 29 to properly position same and to insulate one from the other and all from the standards themselves. Field rings 34 insure an even potential drop over the distance between iilament 22 and accelerating electrode 32, and to further this purpose there may be connected between accelerating electrode 32 and each adjacent field ring a resistor 36 and capacitor 37. The existence of a parallel electric held adiacent filament 22 is insured by the positioning of same within the aperture in conducting plate Zit in line with the surface thereof adjacent accelerating electrode 32, and plate 28 may also be electrically connected in the cld ring circuit as by a resistor and capacitor 36 and 37 and through a suitable conductor and leadthrough insulator in the envelope ange to an external electrical terminal 3S.

lons formed at filament 22 are attracted therefrom by accelerating means 21 to form an ion beam directed axially into envelope 11, and this ion beam is hunched by the action of a varying voltage applied between accelerating electrode 32 and face plate 31. Here again it is necessary to guard against radial field components for accurate time-of-ight mass separation requires parallel ion traiectory, and thus there are provided a pair of transparent grids 41 and 42 in electrode 32 and plate 3l, respectively. These grids, as best shown in Fig. 4, may be similarly constructed and grid 41, for example, includes an annular support ring 43 and a plurality of fine parallel wires 4d secured thereto. Grids 41 and 42 are secured in the apertures in electrode 32 and face plate 31, respectively, perpendicular to the axis of envelope 11 and are disposed as shown with the wires thereof in very close proximity. Because of the close spacing required and the exacting electric eld requirements, it is particularly important that the wires of grids 41 and 42 be parallel and, thus, it is desirable to use an electronic grid such as that disclosed in my copending application Serial No. 249,374, led October 2, 1951 and entitled Electronic Grid. Face plate 31 and attached grid 42 are electrically joined to envelope 11 which is, in turn, electrically grounded, and the bunching voltage is thus applied between accelerating electrode 32 and ground, as noted in more detail below.

At the opposite end of envelope 11 from ion transmitter 13 is disposed receiver structure 14 within evacuated box 17. Receiver 14 includes a pair of parallel gating grids 51 and 52 disposed across the end of envelope 11 and perpendicular to the axis thereof. Grids 51 and 52 are disposed in very close proximity and are maintained in this spaced relation by an insulating ring 53. Grid 51, nearest envelope 11, is secured to an apertured end plate 55 mounted upon the ilanged end of envelope 11 about the end thereof, and grid 52, furthest 'i from envelope 11, is secured to the side of an annular plate 54 nearest envelope 11. Suitable securing means, such as bolts S6, are provided to connect the eut-ire gating grid structure together by extending through palte Sfiand insulating ring 53 into engagement with end plate 55, 'Y

and electrically insulating cylinders may be provided about bolts 56 to prevent electrical connection of elements of the gating grid structure. An electrical conductor 57 contacts annular plate 54 and makes electrical connection via a lead-through insulator in the flange about the end of envelope 11 to a terminal 5S exterior to envelope 11.

0n the opposite side of the gating grid structure from envelope 11, there is provided a repeller means or grid 59 which may include an open-ended box 61 which has an outwardly extending ange about the open end thereof. Repeller box 61 is disposed with the open end thereof facing or confronting the gating grid structure and is maintained in position by bolts 62 secured to annular plate 54 of the gating grid structure and is spaced and insulated therefrom by cylindrical insulators about bolts 62. The end of repeller box 61 has an aperture formed therein symmetrically about an extension of the axis of envelope 11, and there is formed on or secured to the back of repeller box 61 an ion dellecting electrode 63 which extends away from box 61 and is bent to be in part parallel to the end of box 61 at a short distance therefrom in line with the aperture in box 61. Repeller means 59 has an electrical lead connected to the box 61 thereof which extends to a lead-through insulator in the CII anged end of envelope 11 to an electrical terminal 64 exterior of 4envelope 11, and terminal A64 is adapted for connection to a positive voltage source whereby rcpeller means 59 including box 61 and deilecting electrode 63 are charged to repel ions. There may be provided in the aperture in the end of repeller box 61 a grid structure; however, the charging of box 61 closely approximates a grid action so that same is not necessary and for this reason repeller means 59 may be considered to be a grid and is so termed herein.

Exterior to rcpeller grid box 61 there is disposed an ion collector 66 at approximately a ninety degree angle to the aperture in repeller box 61 in position to receive ions passing through said aperture and dellected 'oy deilecting electrode 63. Collector 66 may comprise any desired combination of receiving or collecting and ir.- dicating apparatus and there is illustrated an electron @missive cathode structure A67 responsive to impinging ions and directing electrons produced .thereat into an eicetron multiplier 68, as for exam-ple a beryllium-copper multiplier tube. Suitable indicating apparatus may be connected to the output of multiplier 68 and collector 66 is connected by an electrical conductor and lead-through insulator in the wall of box 17 to a .terminal 69 exterior to box 17 whereby collector 66 may be suitably charged to attract and retain ions passing through repeller grid 59.

r[here may be further provided within envelope 11 an auxiliary ilament 71 which is disposed adjacent a wall of envelope 11 and which operates to produce electrons for neutralizing space charge effects within the apparatus. Filament 71 may consist only of a single wire, such as tungsten or the like, which is connected to external terminals 72 by lead-through insulators in the wall of envelope 11.

With regard to the electrical circuits of the invention, reference is made to Fig. 6 wherein like numerals are employed for parts corresponding to elements above described in connection with Figs. 1-5. The filament wire 22 of ion source 19 is energized by a suitable power supply 73 connected through a transformer 74 across ion source terminals 26 whereby filament wire 22 may be heated by large electrical currents liowing therethrough. Ion accelerating voltage is supplied by a direct current power supply 76 shown for purposes of illustration as a battery and having the negative terminal thereof grounded. The positive terminal is connected through a choke or impedance coil 77 to ion source plate 28 and to the midpoint of a voltage divider connected between iilament terminals 26. The accelerating voltage circuit is completed through interconnected field rings 34 and accelerating electrode 32 which is electrically grounded at the terminal 33 thereof through an impedance coil 78. By this circuit lilament 22 is maintained at a positive potential with respect to grounded accelerating electrode 32 so that the relatively negative potential thereon attracts ions from filament 22. The potential employed for above-noted ion bunching process is applied from a sawtooth voltage generator 79 to the ion transmitter as a whole. Generator 79 is connected to accelerating electrode terminal 33 and through a blocking capacitor 80 to ion transmitter terminals 38 and 26, so that the entire ion transmitter, with the exception of grounded faceplate 31, varies in potential with the sawtooth voltage output of generator '79. As face plate 31 is electrically grounded, there is thus produced between accelerating electrode 32 and face plate 31, and consequently between bunching grids 41 and 42 attached thereto, a sawtooth voltage. Preferably the voltage output of generator '79 is of a positive polarity so that ions from source 19 are accelerated between bunching grids 41 and 42, this acceleration effect becoming increasingly strong as the sawtooth voltage rises. The transit time of ions between the grids 41 and 42 is quite small compared to( the duration of individual sawtooth voltage pulses impressed between these grids whereby ions arriving between the grids later in the rising voltage part of the cycle are additionally accelerated to produce an ion bunching effect.

At the receiver end of the apparatus, first gating grid 51 is electrically grounded to envelope 11 and second gating grid 52 has a pulsed voltage applied thereto for producing an ion accelerating field between the gating grids 51 and 52. This pulsed voltage is produced by a pulse forming circuit 81 which is driven by a xed frequency oscillator 82 and which has the output thereof connected to gate grid terminal 58. Pulse forming circuit 81 is also connected to sawtooth generator 79 for impressing the output thereof thereupon so as to operate the sawtooth generator in synchronism with the gate pulse. A positive repeller grid voltage is provided by a direct current power supply 83 which may have the positive terminal thereof connected to repeller grid terminal 64 and the negative terminal thereof to the midtap of the voltage divider between filament terminal 26. This circuit maintains repeller grid 59 at a positive potential with respect to ion transmitter 13 so as to repel all ions not additionally accelerated by gating grids 51 and 52. The collector 66 is maintained at a highly negative potential by means of a voltage supply 84 connected between ground and collector 66 so as to attract all ions passing through repeller grid 59, and auxiliary filament 71 is energized to produce electrons within envelope 11 by connection through a transformer to a suitable source 86 of alternating current.

Considering the operation of the invention, the material to be analyzed is rst placed upon ion source {ilament 22, as for example in liquid form, and the ion transmitter assembled in closing relation to the end of envelope 11 which is then evacuated through pipe 18. Energization of ion source 19 from source 73 then heats filament 22 to ionize the material thereon and energization of the ion transmitter by power supply 76 produces an axial electric field attracting ions from filament 22 into a beam directed into envelope 11. With oscillator 82 operating, pulse forming circuit 81 produces very sharp negative voltage pulses which trigger sawtooth voltage generator 79 and the positive sawtooth voltage output thereof is impressed upon the ion transmitter 21. This sawtooth voltage is effectively blocked from electrical ground by coil 78 which provides the direct current ground line for accelerating electrode 32 and there thus appears between bunching grids 41 and 42 a sawtooth ion accelerating voltage. During each cycle of sawtooth voltage ions arriving between bunching grids 41 and 42 at progressively later times receive correspondingly greater acceleration so that there emerges into envelope 11 an ion beam divided into bunches.

During the transit of ions through envelope 11, the ions of each bunch will separate because of the greater velocity of the ions having lesser mass so that each bunch of ions arriving at gating grid 51 actually comprises a plurality of groups of ions with each group consisting of ions of only a single mass. This separation will be seen to be a time-of-ight phenomena for it is in time that these ions are separated, i. e., they arrive at definite time intervals; however, for practically sized apparatus the time separation is very slight so that it is necessary for the gating or discriminating portion of the invention to operate with a high degree of electiveness and rapidity in order to provide good resolution. In the present invention this is accomplished by the impression upon second gate grid 52 of a very sharp negative voltage pulse from pulse forming circuit 81 which may have a duration of the order of one-iiftieth of a microsecond. This negative voltage pulse creates an ion accelerating field between gating grids 51 and 52 which thereby accelerates certain of the ions of each bunch and the ions so accelerated thus attain a suicient velocity to overcome the repulsive force of repeller grid structure 59. The ion bunches are continually formed by the sawtooth bunching voltage so that they follow each other at con- F stant time intervals to form an intermittent or varying intensity ion beam and thus the pulsed gating voltage continuously repeats at the same frequency as the bunching voltage to accelerate predetermined ions of each group leaving ion transmitter 13.

Considering an individual group of ions bunched at bunching grids 41 and 42, it will be seen that the gating pulse must occur at some time after the bunching voltage in order to operate upon ions of the group hunched by the corresponding sawtooth voltage pulse or wave. The ion transit time between bunching and gating grids is a function of the ionic mass and charge, the distance between grids, and the accelerating voltage so that with a fixed distance between grids and a particular ion to be collected the time may be set by the accelerating voltage and this is preferably accomplished so that the ion transit time is a multiple of the bunching voltage frequency in order that no time lag need be electronically supplied. Variation of the particular ion to be operated upon by the gating grids may be accomplished either by changing the voltage frequency, i. e., of oscillator 82, or by changing the amplitude of the accelerating voltage, and the latter is employed in the illustrated embodiment of the invention.

The repeller grid structure 59, as noted above, is positively charged with respect to ion transmitter 13 by power supply 83 and thus repeller grid structure 59 will repel ions having only the energy imparted thereto by accelerating means Z1. Ions of a predetermined mass which are accelerated between gating grids 51 and 52 obtain sufficient energy to pass through repeller grid 59 and these ions are deected by the positive potential of repeller box 61 toward an extension of the axis of enevelope 11 so as to pass through the aperture in the back Wall of box 61. Ions passing through this aperture are deiiected downward by the positive potential of deliecting electrode 63 which electrode is disposed normal to the ion path as well as above same. These deflected ions are attracted to collector 66 by the negative potential thereon from power supply 84 and the ions striking cathode 67 of collector 66 produce electrons which may be amplied in tube 68 and the resultant signal applied to suitable indicating means, not shown. The particular conguration of repeller grid structure 59 is highly advantageous in that it prevents other than ions of a predetermined mass from reaching collector 66. Thus, ions of other masses which receive no acceleration from gating grids 51 and 52 are repelled and thus reverse direction to eventually strike some other surface in the apparatus. Neutral particles possibly present in the receiver 14 are prevented from reaching collector 66 by the curved path necessary to follow and secondary electrons formed in the receiver area will, at most, strike deiiecting electrode 63 if they pass through the repeller aperture as deecting electrode 63 has a positive potential which attracts electrons. Consequently, there is substantially no possibility of any particles other than ions of a particular predetermined mass from reaching collector 66 and consequently the error signal is minimized.

Itis to be appreciated that numerous other modifications of the invention are possible, and, for example, a bombardment source may be employed rather than the thermal one illustrated. Also, various types of collectors may be used, as for example the one shown in Fig. 7, wherein a pair of collector plates 91 and 92 are employed, these plates being disposed at an angle to each other on opposite sides of an extension of the envelope axis and sloping away from envelope 11. Repeller grid structure 59 is preferably modified by enlarging the aperture in the back of repeller box 61 and inserting a grid 93 therein and insulating standards may be employed to mount repeller box 61 and collector plates 91 and 92 within box 17', as shown. The electrical circuit associated with the modified collector includes a multiple switch 94 which may be relay operated through a manual switch 95 or auto matic control from a suitable power source. Switch 94 includes two contactors 96 and 97 connected to collector plates 91 and 92, respectively, and adapted to be moved together between first and second positions. In first or upper position, contacter 96 grounds collector plate 91 and contactor 97 connects collector plate 92 to the repeller grid voltage so that collector plate 92 in effect operates as a dellecting electrode directing ions upon collector plate 91. in the opposite or second position, shown in Fig. 7, switch 94 grounds collector plate 92 and con nects collector plate 9i to the repeller grid voltage to direct ions upon collector plate 92. A third contacter 98 is also provided on switch 94 and is connected in the accelerating voltage supply line, as for example between power supply 76 and choke coil 77. In the above-noted first position of switch 94, contacter 9S directly connects power supply 7 to terminal 3S, as shown in Fig. 6y and in the second position contactor 9S connects power supply 76 through resistor-battery combination 99 with provision being made within same to vary the effective battery potential by a plurality of taps, as shown. In the second position of switch 94 the effective accelerating voltage of ion transmitter 2i is thus the voltage of power supply 76 plus the voltage of the combination 99.

The operation of the modified collector of Fig. 7 is quite simple in that in the first or upper position of switch 9d, collector plate 92, repeller grid 93, and box 6l are maintained at the same potential to direct ions of a predetermined mass upon collector plate 9i. In the second or lower position of switch 94, collector plate 91 is maintained at the same potential as the repeller grid structure 59 to direct ions upon collector plate 92 and at the same time contacter 98 inserts an additional accelerating voltage in the circuit so that the ion travel time is changed and ions of a different mass reach the collector. Thus, as switch 94 is operated from first to second positions, there are collected on different collector plates ions of different masses and by the provision of a multitapped battery and variable resistor in the accelerating voltage circuit the ions to be collected on collector plate 92 may be changed. Switch 94 may be relay operated either manually or automatically, as noted above, and the automatic control may well be in accordance with time to bombard the collector plates alternately for predetermined and repeating periods.

A further modification of the invention may be made to eliminate harmonics which result from the fact that ions with a transit time of any integral multiple of the period of the oscillator may be collected. Where only ions of slightly different masses are to be accelerated and collected, substantially no harmonics are found; however, where ions of widely dilerent masses are produced there will occur harmonics resulting from the fact that a multitude of -ion bunches are injected during the period of ion transit time from the bunching to gating grids whereby ions other than those of the desired mass arrive at the gating grids at proper times to be accelerated thereby. While a variety of harmonic elimination metlods may be employed, one found quite successful is the addition to the bunching and gating voltages of a square wave blanking pulse. This blanliing pulse is applied only once every ion transit time between bunching and gating grids so that a single group of ions travel from the bunching grids tothe accelerating grids wherein they receive suf* cient acceleration to reach the collector and at the same time another group is operated upon by the combined bunching and blanking voltage with all intermediate groups of ions receiving only the nominal gating acceleratidn so that they are repelled by the added potential of the repeller grid structure. It will be appreciated that this system reduces the transmission, i. e., the percent of ions transmitted that are collected, and for this reason is employed only when undue harmonics are encountered.

From the foregoing, it will be seen that there is provided by the present invention an improved method and means of analyzing materials electrostatically by time- Cil (ill

of-flight principles. The invention because of the novel method ot detecting the desired ions in the beam of ions provides a very high resolution and much improved transmission. It will be appreciated that numerous modifications and variations are possible within the spirit and scope of the invention, and thus it is not intended to limit the invention to the details of the illustrated preferred embodiment, but rather attention is directed to the following claims for a precise delinition of the invention.

Nhat is claimed is:

l. An improved mass spectrometer comprising an ionv source, an ion accelerating electrode disposed adjacent said source for accelerating a beam of ions therefrom, first and` second pairs of grids disposed in the path of said accelerated ions, an ion collector disposed on the opposite side of said pairs of grids from said ion source, a repeller grid disposed intermediate said pairs of grids and said collector', and power supply means connected to said grids for impressing between said first pair of grids a sawtcoth voltage to bunch ions passing therethrough, for impressing upon said repeller grid a voltage in excess of said accelerating voltage, and for impressing between said second pair of grids a pulsed ion accelerating voltage in synchronism with said bunching voltage whereby ions of a predetermined mass are additionally accelerated between said second pair of grids to overcome the repelling force of said repeller grid and impinge upon said ion collector,

2. An improved mass spectrometer comprising an evacuated envelope, an ion transmitter disposed at one end of said envelope and accelerating ions lengthwise thereof, bunching electrodes disposed adjacent said transmitter for dividing ions ejected therefrom into groups, gating grids disposed at a distance from said buuching electrodes in line with the ion trajectory whereby ions of the same mass reach said grids at the same time, first power supply means periodically impressing between said gating grids an ion accelerating voltage whereby ions of a single mass are accelerated therethrough, ion receiving means disposed on the opposite side of said gating grids from said ion transmitter and including ion repelling means, and second power supply means connected to said repclling means and maintaining same at a positive potential with respect to said ion transmitter for repelling all ions except those accelerated by said gating grids.

3. An improved mass spectrometer comprising an evacuated vessel, an ion source disposed within said vessel for ionizing a material to be analyzed, accelerating electrodes disposed adjacent said ion source for accelerating the ions therefrom into a directed beam, a pair of electrodes having apertures therein aligned with said ion beam, means impressing a periodically varying voltage between said pair of electrodes for bunching said beam into groups of ions whereby each group of said ions in traiectory separates according to ionic mass, a pair of grids disposed in the path of said ion beam, means" impressing between said grids a voltage varying periodically in synchronism with said bunching voltage to accelerate ions of a single mass in each ion group of said beam, an ion collector disposed on the opposite side oi said grids from said bunching electrodes for collecting ions of a single mass, a repeller grid disposed intermediate said grids and said collector, and means irnpressing a voltage on said repeller grid in excess of the potential of said accelerating electrode whereby all ions not accelerated by said grids are repelled from said collector.

4. An improved mass spectrometer comprising an evacuated vessel, an ion source disposed within said vessel, ion accelerating electrodes for transmitting an ion beam from said source, bunching grids disposed in thc path of said ion beam, first power supply means connected to said bunching grids for impressing thereon a sawtooth voltage for bunching ions of said beam into separated groups, a pair of grids disposed in close proximity in the path of said hunched ion beam, second power supply means connected to said pair of grids for impressing a periodic ion accelerating voltage between said grids in synchronism with said sawtooth voltage for further accelerating ions of a single mass in each group of ions passing through said grids, an ion receiver including a repelling electrode and third power supply means connected to said repelling electrode for maintaining same at a positive potential with respect to said accelerating electrodes whereby all ions except those of a single mass further accelerated by said grids are repelled from said receiver and only ions of a predetermined mass impinge thereon.

5. An improved mass spectrometer as claimed in claim 4 further dened by said receiver including a pair of facing collector elements disposed on opposite sides of said ion beam and means simultaneously varying the accelerating voltage and switching the repelling electrode potential between the collector elements whereby ions of diierent mass impinge upon each of said col` lector elements.

6. An improved time-of-ight mass spectrometer comprising an ion transmitter for ionizing a material to be analyzed and projecting an ion beam therefrom by an ion accelerating voltage at a known potential, a pair of bunching grids disposed in the ion beam path, a pair of gating grids disposed in the path of said ion beam on the opposite side of said bunching grids from said ion transmitter, first power supply means connected to said gating grids and to said bunching grids for impressing between said bunching grids and between said gating grids a periodically varying voltage, said voltage period being a function of the separation between bunching and gating grids and the accelerating potential and the mass of the ions to be collected whereby only ions of a single predetermined mass are accelerated by said gating grids, a repeller grid structure disposed on the opposite side of said gating grids from said ion transmitter and having an aperture therein aligned with the ion beam, second power supply means impressing a positive potential upon said repeller grid that is greater than the ion accelerating Voltage and less than the sum of the ion accelerating voltage and the gating grid voltage whereby ions accelerated by said gating grids pass through said repeller grid structure and al1 other ions are repelled therefrom, and an ion collector disposed on the opposite side of said repeller grid structure from said gating grids for receiving ions of a single predetermined mass passing through said repeller grid structure.

7. An improved time-of-ight mass spectrometer as claimed in claim 6 further defined by said repeller grid structure comprising an open-ended electrically conducting box disposed with the open end thereof adjacent said gating grids in alignment with the ion beam, said box having an aperture in the rear wall thereof in alignment with the ion beam and having an angular member extending from the outside of said box adjacent the aperture therein and including a portion disposed parallel to the apertured box wall at a distance from the aperture and across same, electrical connections from said second power supply to said repeller grid structure for maintaining the box and member thereof at a positive potential whereby ions accelerated by said gating grids pass through the aperture in the wall of said box and are deiiected by the positively charged member portion adjacent said aperture, and said ion collector being disposed exterior to said box and out of alignment with said box aperture in position to receive ions deflected by said box member whereby random charged and neutral particles are precluded from impinging upon said collector.

8. An improved time-of-ight mass spectrometer comprising an ion transmitter projecting into an evacuated space an ion beam comprising separated groups of ions whereby said groups separate according to the ion mass, accelerating means disposed in the path of said ion beam, power supply means connected to said accelerating means for impressing thereon an ion accelerating voltage varying periodically in synchronism with the transmittal of ion groups in said ion beam whereby only ions of a single mass are accelerated thereby, ion repelling means including an apertured electrode disposed on the opposite side of said ion accelerating means from said ion transmitter and having a positive potential impressed thereon for repelling all ions not accelerated by said accelerating means, said repelling means including a deflecting electrode electrically connected thereto and extending from the ion exit side of the electrode thereof in alignment with the aperture therein and at an angle to the direction of ion traverse whereby ions passing through said repeller means are deflected, and ion collecting means disposed on the ion exit side of said repeller means in alignment with the deected ion beam for collecting ions of a single mass passing through said repeller means.

9. An improved time-of-iiight mass spectrometer as claimed in claim 8 further defined by a variable output power supply connected to said ion transmitter for varying the velocity of ions projected therefrom and thereby controlling the ion transit time whereby ions of different mass are accelerated by said accelerating means and impinge upon said collector.

References Cited in the file of this patent UNITED STATES PATENTS 2,606,291 Wilson Aug. 5, 1952 2,612,607 Stephens Sept. 30, 1952 2,633,539 Altar Mar. 3l, 1953 2,642,535 Schroeder June 16, 1953 2,648,009 Robinson Aug. 4, 1953 

1. AN IMPROVED MASS SPECTROMETER COMPRISING AN ION SOURCE, AN ION ACCELERATING ELECTRODE DISPOSED ADJACENT SAID SOURCE FOR ACCELERATING A BEAM OF IONS THEREFROM, FIRST AND SECOND PAIRS OF GIRDS DISPOSED IN THE PATH OF SAID ACCELERATIONIONS, AN ION COLLECTOR DISPOSED ON THE OPPOSITE SIDE OF SAID PAIRS OF GIRDS FROM SAID ION SOURCE, A REPELLER GRID DISPOSED INTERMEDIATE SAID PAIRS OF GIRDS AND SAID COLLECTOR, AND POWER SUPPLY MEANS CONNECTED TO SAID GIRDS FOR IMPRESSING BETWEEN SAID FIRST PAIR OF GIRDS A SAWTOOTH VOLTAGE TO BUNCH IONS PASSING THERETHROUGH, FOR IMPRESSING UPON SAID REPELLER GRID A VOLTAGE IN EXCESS OF SAID ACCELERATING VOLTAGE, AND FOR IMPRESSING BETWEEN SAID SECOND PAIR OF GRIDS A PULSED ION ACCELERATING VOLTAGE IN SYNCHRONISM WITH SAID BUNCHING VOLTAGE WHEREBY IONS OF A PREDETERMINED MASS ARE ADDITIONALLY ACCELERATED BETWEEN SAID SECOND PAIR OF GRIDS TO OVERCOME THE REPELLING FORCE OF SAID REPELLER GRID AND IMPINGE UPON SAID ION COLLECTOR. 